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Open Loop and Closed Loop

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ccw27

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Just a quick question,

What is the relationship between open loop unity gain frequency and closed loop 3dB bandwidth?

Thanks
 

There is no exact general relationship. But for systems which have a dominant pole, the product of the closed loop gain and the 3dB bandwidth is approximately the open loop unity gain bandwidth. (can be proved for single pole systems)
 

Just want to clear some doubt. So normally when designing Op-amp alone, you check stability by connecting it as unity gain amplifier (β=1) and simulating the Loop Gain (βH), but βH=H. So for a differential Op-amp you can basically simulate Open Loop Gain with the output loading.

But what if you are designing a OP to be used for LPF and you know the feedback resistors and capacitors (on chip) to be used. Do you simulate Loop Gain or Open Loop Gain or first Open Loop Gain and then Loop Gain?

Thanks
 

Although there is some method called return ratio to calculate the loop gain by hand. Simulating the amplfifer's loop gain is hard to be obtained directly in hspice.

However, you can follow Paul Gray'e method to break the loop and simulate it. Refer to his text book and you will find it.
 

But for current-feedback opamp, the -3dB frequency depends on the feedback resistor.
 

"But what if you are designing a OP to be used for LPF and you know the feedback resistors and capacitors (on chip) to be used. Do you simulate Loop Gain or Open Loop Gain or first Open Loop Gain and then Loop Gain?
u must check stability in open & closed loop
 

It is not always so. In the voltage feedback, it's OK,but in the current feedback system,it's not correct
 

allenyyq said:
It is not always so. In the voltage feedback, it's OK,but in the current feedback system,it's not correct
Click to expand...
This confused me and which is not correct in the current feedback system?
My opinion is that the 3db bandwidth of close loop system is BW/As.(Bw is the unit gain bandwidth of open loop system and As is the Gain of close loop system)
 

the gain bandwith product is fu\
it is a constant
either in open loop or close loop
 

layes2 said:
the gain bandwith product is fu\
it is a constant
either in open loop or close loop
Click to expand...
This is correct.
But the 3db BW is different between open loop system and closed-loop system.
 

there difference betwween current feedback and voltage feedback opamp
 

I want to know a opamp is used to a close loop,the proportion of feedback resistor is low,&the whole gain of the circuit is low?
 

layes2 said:
the gain bandwith product is fu\
it is a constant
either in open loop or close loop
Click to expand...

It is not always so. In the voltage feedback, the gain bandwith product is constant,but in the current feedback system,it's not constant
 

no relation ship
the gain *bandwith are the same
 

Current mode or voltage mode?
 

voltage-feedback:
gain*bw=constant

current-feedback:
-3dB bw=constant
 

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